A conserved boundary-specific NAC transcription factor targets various regulators to modulate rice aerial architecture

The mechanisms underlying boundary development in monocots remain poorly understood. In dicots, the formation of boundary regions is crucial for organ separation and the initiation of axillary meristems (AMs). Here, we isolated a rice (Oryza sativa) mutant exhibiting defects in panicle branching, tiller development, and leaf collar formation, which we designated abnormal boundary development (abd). Map-based cloning revealed that the mutated gene encodes a conserved NAC transcription factor, CUP-SHAPED COTYLEDON 1/NO APICAL MERISTEM (OsCUC1/NAM), which is an ortholog of Arabidopsis thaliana CUC1, CUC2, and Petunia hybrida NAM. The rice OsCUC1 homolog OsCUC3 is the ortholog of Arabidopsis CUC3. OsCUC1 and OsCUC3 expression is specific to boundary regions and these genes function redundantly in shoot organogenesis. Overexpression of OsCUC1 induces ectopic collar formation at the margins of the leaf blade and accelerates tiller formation. Furthermore, OsCUC1 directly binds to the promoters of MONOCULM1 (MOC1) and LIGULELESS1 (OsLG1) to regulate rice branching and leaf collar formation, respectively. Additionally, OsCUC1 directly regulates several shattering-related genes in the abscission zone, including QTL OF SEED SHATTERING IN CHROMOSOME 1 (qSH1), SHATTERING ABORTION1 (SHAT1), and OsYABBY2/Shattering1 (OsSh1). OsCUC3, also affects rice shattering. Therefore, we propose that OsCUC1 regulates boundary establishment during various developmental stages and modulates shoot architecture in rice.